Melt 1 tablespoon butter in a small skillet over medium-high heat. Add onion, and saute 2 minutes or until tender; set aside.
Combine baking mix and half of cheese in a large bowl; make a well in center of mixture.
Stir together onion, egg, and milk, blending well; add to cheese mixture, stirring just until dry ingredients are moistened. Spoon into lightly greased muffin pans, filling two-thirds full. Sprinkle evenly with remaining cheese and sesame seeds; drizzle with butter.
Bake at 400° for 15 to 20 minutes or until golden.
NOTE: Substitute mini muffin pans for regular pans, if desired. Bake at 400° for 12 to 14 minutes or until golden. Makes 1 1/2 dozen mini muffins
*1 cup (4 ounces) shredded Monterey Jack cheese with peppers may be substituted for Cheddar cheese.